A number of prior art processes have been proposed for the thermal decomposition of substituted ureas particularly arylureas at temperatures ranging from 135.degree.C. to 500.degree.C. to produce the corresponding amines and isocyanates. Such processes are generally carried out in the vapor phase and facilitated by the presence of phosphorus pentoxide, carbonyl chloride (phosgene) or hydrogen chloride. The recombination of the dissociation products which can occur readily on cooling has generally limited the recovery of the isocyanates from these types of reactions giving low yields.
The thermal decomposition of substituted ureas in liquid phase at high temperatures has also be reported. In most cases the liquid phase reaction at high temperatures resulted in low yields of isocyanate and were further complicated by secondary reactions resulting in the undesirable formation of polymerization products such as tars and resins. French Pat. No. 1,473,821 (1967) discloses a method for the liquid phase (low temperature) pyrolysis of substituted ureas to isocyanates and amines using a particular class of solvents and at temperatures of 200.degree.C. and below, with reaction times ranging from 6 to 35 hours to give moderate yields of the isocyanate. The French patent further discloses that temperatures above 250.degree.C. are to be avoided, not only because of high thermal requirements, but because of the secondary reactions giving mediocre yields of the isocyanates and impure products.
Isocyanates are compounds of significant industrial importance and have become large volume items of commerce with well-known uses in the polymer and coatings field with tailored properties.
Contrary to the teachings of the prior art, it has surprisingly been discovered that di- and tri-substituted ureas can be thermally decomposed to give excellent yields of isocyanates which can be obtained within relatively short reaction times and at elevated temperatures, i.e., temperatures of 230.degree.C. and higher and at the same time essentially avoid the problems of secondary reactions and recombination of the dissociation products encountered by prior processes.